To provide the greatest listening pleasure, an acoustic loudspeaker must meet several basic requirements. First, a loudspeaker must accurately reproduce very low frequencies, such as bass notes below 40 Hz, which are felt more than heard by most listeners. Second, loudspeakers must accurately reproduce overtones of high frequencies. Third, a loudspeaker should have a relatively flat frequency and phase response over the full range of audible frequencies, i.e., from approximately 30 Hz to 20,000 Hz, in order to produce high-fidelity sound. Fourth, to provide a wide dynamic range, a loudspeaker must handle signals with power sufficient to reproduce low frequencies at loud volumes without distortion to the sound or damage to the speaker.
In addition to the ideal frequency and phase response characteristics of a loudspeaker, a system of multiple loudspeakers should recreate whatever spatial illusions are contained in the source material. For example, most music sources are encoded for stereo reproduction using two channels. Two, spatially-separated and phase-synchronous infinitesimal point sources of acoustic energy theoretically provide the best stereo imaging, because such point sources can create the illusion of sound originating from any point along a line extending through both point sources. Therefore, a loudspeaker system should imitate as closely as possible two infinitesimally small point sources of acoustic energy.
A conventional acoustic transducer has a relatively stiff or rigid diaphragm which reciprocates along a linear axis. For reproducing low frequencies, the diaphragm has preferably a concave, cone shape. For high frequencies, it may be flat or convex. To vibrate the diaphragm, an electrical signal representing the sound wave to be reproduced flows through a coil mechanically connected to the diaphragm. The coil is situated within a fixed magnetic field, causing the coil to reciprocate with changes in the current. The coil is formed from one or more lengths of wire wrapped around a support structure. Typically, the edges of the diaphragm are attached to a basket shaped frame using a compliant, slightly resilient, material. The coil is centered within a gap referred to as a “flux gap,” formed between a cylindrically shaped pole and a donut-shaped magnet assembly.
To provide the most accurate sound reproduction, the movement of the coil in response to the electrical signal and the coupling of the movement of the diaphragm to the air in response to the movement of the coil must be linear. Unfortunately, the responses of these elements to the sound signal are rarely totally linear, especially over the entire audible range. The diaphragm couples the mechanical energy of the moving coil to the air, thereby causing the air to vibrate and setting up acoustic waves. At lower frequencies, the diaphragm can be thought of as behaving like a simple mechanical piston pushing volumes of air. At low frequencies, a lot of power is required to push large volumes of air, particularly at loud volumes. Therefore, to sound low notes with great volume a speaker must be capable of handling a lot of power, mechanical stresses from the strong electromagnetic forces and resulting heat.
For good low frequency response, a driver is needed which is mechanically strong and powerful in order to move larger amounts of air. Thus, a stiffer diaphragm with a large surface area is preferred. However, a large, stiff diaphragm means more structure, and thus more mass. More mass means less efficiency, and thus more power to reproduce the same loudness. More power means that a more massive coil is required to handle the mechanical and thermal stresses resulting from the power. However, more mass in the moving parts inhibits the driver's ability to reciprocate at higher frequencies. Also, it is more difficult to control coupling of the movement of the coil to the air through a large diaphragm and its natural resonances. A smaller diaphragm could be used to sound bass notes, but a longer throw or stroke of the coil would be required to move the same amount of air. However, a longer stroke necessitates either a magnetic field of greater magnitude or a longer coil in order to provide a sufficiently high electromotive force (EMF). Furthermore, a greater coil length means greater induction. Thus, the length of the coil is limited. A long stroke also requires the coil to move at a higher velocity. Higher velocities will create a higher back EMF, which resists travel of the coil and ultimately limits the ability of the driver to reproduce low frequencies.
Attempts have been made to accommodate the demands of high and low frequencies in a single, broad band acoustic driver, particularly in the area of reducing the mass of the moving parts of the driver. For example, as shown in U.S. Pat. Nos. 4,115,667 and 4,188,711 of Babb, the conventional rear suspension for the coil is replaced with a low friction bearing made of TEFLON®. The bearing is formed at the bottom of the coil, opposite of where it connects to the diaphragm, and encircles and rides on the post. The coil remains centered within the gap without the extra mass of the rear suspension and its spring forces interfering with movement of the coil. The coil therefore can move more freely and accelerate faster, which aids in moving the coil long distances when using a longer throw coil to sound bass notes. A low friction bearing can also be added around the circumference of the top end of the post. Lightweight, stiff metal alloys have been used to form diaphragms. Coil forms (structures for supporting windings of coils) have been made from high strength, thermally resistant materials such as KAPTON®. To provide a low mass, compliant suspension for the diaphragm, a stamped synthetic foam having a very low density with good dampening and resonance characteristics is used.
However, a coil undergoes great mechanical stress from the EMF generated by the magnet and the current running through the coil, as well as great thermal stress from the substantial heat generated when large currents flow through the coil during reproduction of loud notes. Despite the use of lightweight, stiff materials, a low mass coil capable of sounding both high and low frequencies will naturally tend to be weaker and thus more easily deformed by the mechanical and thermal stresses present during reproduction of high power sounds. A low mass coil also cannot store heat for later dissipation. Thus, during extended periods of loud notes, a low mass coil will tend to get very hot and possibly damaged. Furthermore, TEFLON® is not structurally strong and tends to shrink in heat, thus resulting in increased drag of the coil's bearing on the post and deformation under high thermal and mechanical loads. A deformed coil cannot sound notes as accurately and will tend to rub against the walls defining the flux gap, causing noticeable distortion of low notes and extraneous noise at midrange frequencies.